The Sahara is almost as large as the continental
United
States, and is larger than Australia. The
Sahara has an intermittent history that may go back as much as 3
million years. Some of the sand dunes can reach 180 meters (600
feet) in height. Its name comes from the Tamajaq Tuareg language word
Tenere,
which means the desert, translated into the Arabic it gave Sahara
"desert": (صَحراء), "ṣaḥrā´" (صحراء). But the original name of the
Sahara is: Tinariwen (the
deserts), as for the indigenous people of the Sahara, the Tuareg
people, the Sahara is not a desert, but many deserts, distinct and
different in nature and lanscapes.

It has been reported that the Sahara is expanding
south by as much as 30 miles per year, overwhelming degraded
grasslands, taking over the Sahel, the dry
tropical savanna that
has defined the Sahara's southern limit. Global
warming and poor farming methods have been given as possible
causes. One report states that all of Africa could eventually
become a massive desert. This spreading of deserts is known as
"desertification," and
the phenomenon is occurring in other desert areas worldwide.

The desert landforms of the Sahara are shaped by
wind (eolian) or by
occasional rains, and include sand dunes and dune fields or sand
seas (erg),
stone plateaus (hamada),
gravel plains (reg), dry valleys (wadi), and salt flats
(shatt or chott). Unusual landforms include the Richat
Structure in Mauritania.

Most of the rivers and streams in the Sahara are
seasonal or intermittent, the chief exception being the Nile River,
which crosses the desert from its origins in central Africa to
empty into the Mediterranean. Underground aquifers sometimes reach the
surface, forming oases,
including the Bahariya,
Ghardaïa,
Timimoun,
Kufrah, and
Siwah.

The center of the Sahara is hyper-arid, with
little vegetation. The northern and southern reaches of the desert,
along with the highlands, have areas of sparse grassland and desert
shrub, with trees and taller shrubs in wadis where moisture
collects.

To the north, the Sahara reaches to the
Mediterranean Sea in Egypt and portions of
Libya, but in
Cyrenaica
and the Magreb, the Sahara
borders
Mediterranean forest, woodland, and shrub ecoregions of
northern Africa, which have a Mediterranean
climate characterized by a winter rainy season. According to
the botanical criteria
of Frank White and geographer Robert
Capot-Rey, the northern limit of the Sahara corresponds to the
northern limit of Date Palm
cultivation (Phoenix dactylifera), and the southern limit of
Esparto
(Stipa tenacissima), a grass typical of the Mediterranean climate
portion of the Maghreb and Iberia. The northern limit also
corresponds to the 100 mm
isohyet of annual precipitation.

To the south, the Sahara is bounded by the
Sahel, a belt
of dry
tropical savanna with a summer rainy season that extends across
Africa from east to west. The southern limit of the Sahara is
indicated botanically by the southern limit of Cornulaca
monacantha (a Chenopodiaceae),
or northern limit of the Cenchrus
biflorus, a grass typical of
the Sahel.
According to climatic
criteria, the southern limit of the Sahara corresponds to the 150
mm isohyet of annual precipitation (keeping in mind that precipitation
varies strongly from one year to another).

Climate history

The climate of the Sahara has undergone
enormous variation between wet and dry over the last few hundred
thousand years. During the last ice age, the
Sahara was bigger than it is today, extending south beyond its
current boundaries. The end of the ice age brought better times to
the Sahara, from about 8000 BC to 6000 BC, perhaps due to low
pressure areas over the collapsing ice sheets to
the north. Once the ice sheets were gone, the northern part of the
Sahara dried out. However, not long after the end of the ice
sheets, the monsoon,
which currently brings rain
only as far as the Sahel, came further north and counteracted the
drying trend in the southern Sahara. The monsoon in Africa (and
elsewhere) is due to heating during the summer. Air over land
becomes warmer and rises, pulling in cool wet air from the ocean,
which causes rain. Paradoxically, the Sahara was wetter when it
received more solar insolation in the summer.
Changes in solar insolation are caused by changes in the Earth's
orbital
parameters (9,000 years ago the Earth's axis had a stronger
tilt than it does presently, and perihelion occurred
at the end of July).

By around 3400 BC, the monsoon retreated south to
approximately where it is today, leading to the gradual rather than
abrupt desertification of the
Sahara. The Sahara is currently as dry as it was about 13,000 years
ago. Precipitation, while rare, is not unknown. Half of the Sahara
receives less than 2 cm of rain a
year, with the rest receiving up to 10 cm a year. The rainfall
happens very rarely, but when it does it is usually torrential when
it occurs after long dry periods, which can last for years.

Presently, the climate is changing, and the
Sahara is expanding southward by about 30 miles per year.

Ecoregions

The Sahara comprises several distinct ecoregions, whose variations
in temperature, rainfall, elevation, and soils harbor distinct
communities of plants and animals. According to the
Worldwide Fund for Nature (WWF), the ecoregions of the Sahara
include:

South Saharan steppe and woodlands: The South Saharan steppe
and woodlands occupy a narrow band running east and west between
the hyper-arid Sahara and the Sahel savannas to the
south. Movements of the equatorial
Intertropical Convergence Zone (ITCZ) bring summer rains during
July and August which average 100 to 200 mm, but vary greatly from
year to year. These rains sustain summer pastures of grasses and
herbs, with dry woodlands and shrublands along seasonal
watercourses. The ecoregion covers 1,101,700 square kilometers
(425,400 square miles) in Algeria, Chad, Mali, Mauritania, and
Sudan.

West Saharan montane xeric woodlands: Several volcanic
highlands in the western portion of the Sahara provide a cooler,
moister environment that supports Saharo-Mediterranean woodlands
and shrublands. The ecoregion covers 258,100 square kilometers
(99,700 square miles), mostly in the Tassili-n-Ajjer
of Algeria, with smaller enclaves in the Aïr of Niger, the
Dhar
Adrar of Mauritania, and the Adrar des
Iforas of Mali and Algeria.

Saharan
halopytics: Seasonally-flooded saline depressions in the Sahara
are home to halophytic, or salt-adapted,
plant communities. The Saharan halophytics cover 54,000 square
kilometers (20,800 square miles), including the Qattara
and Siwa
depressions in northern Egypt, the Tunisian
salt lakes of central Tunisia, Chott
Melghir in Algeria, and smaller areas of Algeria, Mauritania,
and Western Sahara..

Fauna

Dromedary
camels and goats are
the most domesticated animals found in the Sahara. Because of its
qualities of sobriety, endurance and speed, the dromedary is the
favorite animal used by nomads.

The monitor
lizard. It has been suggested that the occasional habit of
varanids to stand on their two hind legs and to appear to "monitor"
their surroundings led to the original Arabic name waral ورل, which
is translated to English as "monitor".

Sand
vipers, which average less than 50 cm in length. Many have a
pair of horns, one over each eye. Active at night, they usually lie
buried in the sand with only their eyes visible. Bites are painful,
but rarely fatal.

The addax, a large
white antelope, is a
threatened species. Adapted to the desert, they can remain months
without drinking, even a whole year.

The Saharan cheetah
lives in Niger, Mali and Chad. There remain
only a few hundred cheetahs which are very cautious, avoiding any
human presence, the cheetah flees the sun from April to October. It
then seeks the shelter of shrubs such as balanites and acacias.
They are unusually pale.

History

Egyptians

By 6000 BC predynastic
Egyptians in the southwestern corner of Egypt were herding cattle and constructing large
buildings. Subsistence in organized and permanent settlements in predynastic Egypt by
the middle of the 6th millennium BC centered predominantly on
cereal and animal
agriculture: cattle,
goats, pigs and sheep. Metal objects
replaced prior ones of stone.
Tanning of
animal skins, pottery
and weaving are
commonplace in this era also. There are indications of seasonal or
only temporary occupation of the Al Fayyum in
the 6th millennium BC, with food activities centering on fishing, hunting and food-gathering.
Stone arrowheads,
knives and scrapers are common. Burial items include
pottery, jewelry,
farming and hunting equipment, and assorted foods including dried
meat and fruit. The dead are buried facing due west. Megaliths
found at Nabta Playa
are overt examples of probably the world's first known Archaeoastronomy
devices, predating Stonehenge by
some 1000 years. This complexity, as observed at Nabta Playa, and
as expressed by different levels of authority within the society
there, likely formed the basis for the structure of both the
Neolithic society at Nabta and the Old Kingdom of Egypt.

Phoenicians

The peoples of Phoenicia, who
flourished between 1200-800 BC, created a confederation of kingdoms
across the entire Sahara to Egypt. They generally settled along the
Mediterranean coast, as well as the Sahara, among the peoples of
Ancient
Libya, who were the ancestors of peoples who speak Berber
languages in North Africa and the Sahara today, including the
Tuareg of
the central Sahara.

The Phoenician alphabet seems to have been
adopted by the ancient Libyans of north Africa, and Tifinagh is still
used today by Berber-speaking Tuareg camel herders of the central
Sahara.

Greeks

By 500 BC, a new influence arrived in the form of
the Greeks.
Greek traders spread along the eastern coast of the desert,
establishing trading colonies along the Red Sea coast.
The Carthaginians
explored the Atlantic coast of the desert. The turbulence of the
waters and the lack of markets never led to an extensive presence
further south than modern Morocco.
Centralized states thus surrounded the desert on the north and
east; it remained outside of the control of these states. Raids
from the nomadic Berber
people of the desert were a constant concern of those living on
the edge of the desert.

Urban civilization

Trans-Saharan trade

Following the Islamic conquest of
North Africa in the seventh century CE, trade
across the desert intensified. The kingdoms of the Sahel, especially the
Ghana
Empire and the later Mali Empire,
grew rich and powerful exporting gold and salt to North Africa. The emirates
along the Mediterranean
Sea sent south manufactured goods and horses. From the Sahara itself,
salt
was exported. This process turned the scattered oasis communities into trading
centres, and brought them under the control of the empires on the
edge of the desert. A significant slave trade crossed the desert
(See Arab slave
trade).

This trade persisted for several centuries until
the development in Europe of the caravel allowed ships, first
from Portugal but soon
from all Western Europe, to sail around the desert and gather the
resources from the source in Guinea.
The Sahara was rapidly remarginalized.

European imperialism

At the beginning of the 19th century,
most of the northern Sahara, including most of present-day Algeria, Tunisia, Libya, and Egypt, was part of
the Ottoman
Empire. The Sahel and southern Sahara were home to several
independent states.

Egypt, under Muhammad
Ali and his successors, conquered Nubia (1820-22),
founded Khartoum (1823),
and conquered Darfur (1874).
Egypt, including the Sudan, became a British protectorate in 1882.
Egypt and Britain lost control of the Sudan from 1882 to 1898 as a
result of the Mahdist War.
After its capture by British troops in 1898, the Sudan became a
Anglo-Egyptian
condominium.

Modern times

Egypt became independent of Britain in 1936,
although the Anglo-Egyptian
Treaty allowed Britain to keep troops in Egypt and maintained
the British-Egyptian condominium in the Sudan. British military
forces were withdrawn in 1954.

Most of the Saharan states achieved independence
after World War
II: Libya in 1951, Morocco, Sudan, and Tunisia in 1956, Chad,
Mali, Mauritania, and Niger in 1960, and Algeria in 1962. Spain
withdrew from Western Sahara in 1975, and it was partitioned
between Mauritania and Morocco. Mauritania withdrew in 1979, and
Morocco continues to hold the territory.

The modern era has seen a number of mines and communities develop to
exploit the desert's natural resources. These include large
deposits of oil and
natural
gas in Algeria and Libya
and large deposits of phosphates in Morocco and
Western
Sahara.

Peoples and languages

The Sahara is home to a number of
peoples and languages. Arabic is the most
widely spoken language in the Sahara, from the Atlantic to the Red
Sea. Berber
people are found from western Egypt to Morocco, including the
Tuareg
pastoralists of the central Sahara. The Beja live in
the Red Sea
Hills of southeastern Egypt and eastern Sudan. The Arabic,
Berber, and Beja languages are part of the Afro-Asiatic
language family.